Efforts have therefore been made to electrically discharge helicopters using servo-controlled devices intended to keep the electrical potential of the helicopter at zero volts. Presently available devices for performing this function are not entirely satisfactory.
A major difficulty encountered when designing such a device is measuring the actual potential of the helicopter. For the reasons mentioned above, this measurement must be performed without making contact with the ground (if it is possible to make such contact, there is no need to make a measurement since the contact can be used to discharge the helicopter potential). The electrical field is therefore measured using a field measuring device, e.g. of the windmill type, mounted on the structure of the helicopter.
However, the technical literature on the subject shows that measurements made in this way are spoiled by an unacceptable degree of error whenever the space surrounding the vehicle includes electrically charged particles in suspension. Unfortunately, this is always the case when a helicopter is flying under conditions which tend to give its structure an electrostatic charge. The air may be filled with spray, or sand, or an aerosol of highly charged ice particles. Under these conditions, inputting the error-corrupted measurement to the servo-control system for discharging the helicopter not only has the effect of limiting the minimum potential achievable to not less than about 100 kilovolts, but may also, under some circumstances, actually increase the potential of the helicopter.
That is why proposals have recently appeared in the technical literature which aim at keeping track of the electric charge in the space around the helicopter by calculation in order to determine the potential of the helicopter. The space charge is assumed to be uniform; its value is determined from a sample which may be taken, for example, by means of a suitable sensor; and a microcomputer onboard the helicopter determines the value of the helicopter potential on the basis of the measurement performed by the windmill sensors and by the space charge probe. A device of this type is described in an article by O. TRANBARGER and B. M. DUFF in International Aerospace and Ground Conference on Lightning and Static Electricity, 31, pages 1-34, 21-23 June 1983, Fort Worth, Tex., USA.
This technique is far from perfect since there is no reason for the space charge to be distributed uniformly in the turbulent flow of air round the vehicle. Consequently it appears to be impossible for this method to reduce the potential of the helicopter below a few tens of kilovolts. It may also be observed that this method is cumbersome in that it requires accurate measurement of space charge and the provision of a microcomputer.
Thus, in the present state of the art, and when the sea is rough, it is impossible to perform a rescue at sea by helicopter without exposing both the rescuer and the person being rescued to a dangerous electric shock. More generally, the same is true of rescue operations over land. More generally still, the residual potential may interfere with many types of lifting operation performed by helicopters.
The aim of the invention is to solve this problem by making it possible to reduce the potential difference between the helicopter and ground to a few volts, thereby practically completely eliminating any risk of an electric shock.